freestompboxes.org

Based on conversations I've been having lately with Modman, I decided to pick up an upmarket load box to see what that's all about. I had experimented with the concept a bit way back when I worked for Weber and was making attenuators all day. Lots of people (at least in the TGP scene) think this Suhr one is the hot stuff (or was the hot stuff a few years ago ), so I nabbed one. Inside isn't a surprise: resistors, caps and a couple inductors. Usual things for a reactive load. Anyone familiar with the old old old article on the Aiken Amps site probably thinks this is the least creative thing ever. Well, anyway... here's some pics. Full trace incoming when I have the time to get it on the bench and take it apart. I don't expect to see anything mind-blowing in the circuit.
Interesting that the fan appears to be powered by the input signal... is this considered a part of the magic mojo for the reactive load, or just being cheap not wanting to add a circuit to power it?

Disappointed to see the transformer for the balanced XLR out is so weak and puny. Maybe I'll need to upgrade it to a properly corksniffing manly transformer like a Jensen or Lundahl
Or a Cinemag if I want to be totally hipster.

Right off the bat, I can tell you, the resistive load of the four power resistors in the lid is 7ohms total, with 200w worth of resistors, though since they're not mounted to a substantial heatsink, probably good that Suhr doesn't rate this thing for 200w. Smart move, derating it like that.

If you're wondering how hot these resistors get, here is an art piece I did a couple years ago demonstrating energy waste in internet server farms. I calculated about 200-ish watts with these resistors. They're mounted to server processor heatsinks.

Ive seen in that before and it looked to be a direct implementation of the Aitken circuit.
Are you able to confirm the other values? L of the coils if possible, and the caps?

Optical wrote: ↑15 Jul 2020, 09:25 Ive seen in that before and it looked to be a direct implementation of the Aitken circuit.
Are you able to confirm the other values? L of the coils if possible, and the caps?

I also think this is going to be almost identical to the suggestions in the Aiken article.
I will certainly be measuring and tracing out everything, just give me time to get around to it. Getting ready to move again, so I'm busy right now.

Nice one, will look forward to that

Just for interest, A friend and I did a shoot out with two loads, one a purely resistive and the other " reactive " reamping the same signal
through a Marshall and recording it , running through an ADA speaker sim to test how the load affected the amp transformer ................
not a dramatic difference, I'd be tempted to say the reactive sounded better but that's likely due to bias , and not knowing which was which
you could pick either as being ok! maybe loud & live with the speakers you don't like what you hear but unless you're in the resonate range of the load the transformer doesn't seem to care.

soulsonic wrote: ↑14 Jul 2020, 05:05 Right off the bat, I can tell you, the resistive load of the four power resistors in the lid is 7ohms total, with 200w worth of resistors, though since they're not mounted to a substantial heatsink, probably good that Suhr doesn't rate this thing for 200w. Smart move, derating it like that.

That's because a Marshall 1959 is rated for 100W RMS clean. If you decide you're EVH and forget your Variac that thing will hit up to 190W peak, depending on what revision of the amp it is and how it's biased.

So, realistically, there's about 10W headroom.

A common problem I encountered when doing repairs for Weber was the resistors in the MASS attenuators rattling around loose inside the box because the heat caused the glue to fail. I kept telling him that glue wasn't a good way to mount them in the box...whatever, I'm sure they're still doing it that way

For what it's worth, I mounted them like this, held on with screws:
But this was only for about 30 W IIRC. Could probably find the schematic and simulation if there's interest.

Looking forward to seeing how this differs from the aitkin reactive load. I’ve got most of the parts for it knocking around, I just could never find an appropriate enclosure to start drilling.

Tomrcarre wrote: ↑23 Jul 2020, 15:40 Looking forward to seeing how this differs from the aitkin reactive load. I’ve got most of the parts for it knocking around, I just could never find an appropriate enclosure to start drilling.

If you have to buy an enclosure, maybe one of these would be useful? I found them a couple of years ago while designing a similar but simpler project. All are aluminum except one, most are flanged, listed generally from less to more pricey. Mouser carried them all at that time.

• 1455ZT1601 I used this one, 6 x 3 x 2 inches inside, extruded 1.5 mm U-shape with thinner end plates and cover plate. Clear finish.
• 1590P1F/PFL/P1FL 5.8 x 3 x 1.8 inches inside
• 1590JF 5.5 x 3.5 x 1.7 inches
• 1590EF 7.2 x 4.5 x 3
• 1455KHD1601 6 x 3 x 2
• 1414SCC 6 x 4 x 3
• 1458VB4B 5.9 x 3.8 x 2.8 18 ga steel, AL end plates

Here an article on this topic from a non-existing website: Another circuit diagram:

Finally had a chance to try this thing with my little 6V6 Mystery amp. Sounds good. Haven't done a head-to-head against a resistor load yet, but that would be easy to accomplish. I have a 100w 8ohm resistor dummy load. Easy enough to plug it into the speaker thru jack, which disconnects the internal reactive load.
Anyway, maybe the line output is a little bit anemic with lower-powered amps. I'm thinking a low-power/high-power switch might be a nice mod, to better match the line out with small amps. Seems a little 5 watter would barely light it up. Mystery amp is maybe 15w and has to be turned up high to get much signal, and even then I had to turn the line out volume up to maximum to get a good signal into a line input that expects +4dbu.

If the amp is low power you can run a speaker output direct to your interface (as long as it's plugged into a load as well of course)

Optical wrote: ↑28 Jul 2020, 00:49 If the amp is low power you can run a speaker output direct to your interface (as long as it's plugged into a load as well of course)

You dont agree? the input impedance is high enough to not affect it. Most amp DI outputs are just dividers from the output transformer.

Yes but that dividing is important. Or I suppose it depends on one's definition of "low power": 1W into 4Ω is about as high as you should go.

Optical wrote: ↑30 Jul 2020, 23:24Most amp DI outputs are just dividers from the output transformer.

You're right, but extract signal with the main speaker connected, so the amplifier still work on a reactive load.

Cheers,
Jose